Subversion is actually pretty good for version controlling binary files. It’s just about the only great strength it has over, say, Git (as far as I’m concerned, anyway). See this answer of mine for a more detailed explanation of that assertion.
ImageMagick has a compare command that shows you the pixel-by-pixel difference between two RASTER images. Vectored images might be trickier (unless you can look at an SVG source and immediately see the resulting picture). Unfortunately this doesn’t really help you with Photoshop/Illustrator though.
If you’re working under Windows, TortoiseSVN has an image comparison utility too.